Dual mode stepper motor

ABSTRACT

A method and structure receives a desired rotational speed and/or step frequency for an electric motor. If the desired rotational speed is above a predetermined limit, the method performs closed-loop mode control of coil current of the electric motor by varying the average voltage supplied to the electric stepper motor according to observed feedback current from the current feedback loop connected to the electric stepper motor. To the contrary, if the desired rotational speed is not above the predetermined limit, the method performs open-loop mode control of coil current of the electric stepper motor by setting the average voltage supplied to the electric stepper motor according to values computed from the step frequency, irrespective of the observed feedback current from the current feedback loop.

BACKGROUND AND SUMMARY

Embodiments herein generally relate to structures and methods thatcontrol the speed of electric motors such as stepper motors, and moreparticularly to an improved motor and method that utilizes two modes toreduce vibrations and noise of such motors.

Where perfect speed control is required, hybrid stepper motors are oftenchosen for their precision, in order to minimize vibration. However,even with micro-stepping techniques, vibration continues to be achallenge with popular drive circuitry, as does limitations on theuseable range of speed. This disclosure explains how standard drivecircuit topology can be used along with enhanced control logic toaccomplish very low vibration operation over an unprecedented range ofspeed.

When a stepper motor is used to produce continuous motion, itperiodically updates the electrical signal supplied to the coils. Tokeep the torque constant and the angular increment regular, the coils ofa 2-phase bipolar wound motor (a very common stepper motor arrangement)are driven such that the currents in those coils approximate a sine waveand a cosine wave.

These waveforms, illustrated in FIG. 1, are normally produced using adigital lookup table that is indexed by a counter, and whose output isconverted to an analog signal that controls the driver circuit. The sametable may be used to produce the signals for both motor coils, when themotor coils are indexed by a pair of pointers (or counters) that arealways pointing to positions in the table that are separated by aquarter of the range, hence producing sine and cosine functions of thecounter value. As the counter increments at a given frequency, the motorspins at a corresponding speed. If the counter is decrementing, themotor direction is reversed.

With respect to the effects of vibration, free (unloaded) stepper motorscan be set to move to any position at any torque by controlling the coilcurrents. This is sometimes called the electrical position. However themechanical position of a torque loaded motor will stray from this by apositional error called the load angle, as shown in FIG. 2. The loadangle sets the stage for harmonic motion. Since any imperfection causingnoise at the resonant frequency can build up this oscillation amplitude,what is needed is a damping mechanism.

One exemplary apparatus embodiment herein that provides damping toreduce such oscillation comprises a dual-mode controller operativelyconnected to an electric motor. The controller is adapted to supply apattern of voltage to the electric motor. Further, the controller has aninput/output for receiving a desired rotational speed and step frequencyof the electric motor. A current feedback loop is connected to theelectric motor and the controller.

The controller operates in two different modes, a high-speed,closed-loop mode and a low-speed, open-loop mode. Thus, the controlleris adapted to perform closed-loop mode control of coil current of theelectric motor by varying the average voltage supplied to the electricmotor according to observed feedback current from the current feedbackloop, if the desired rotational speed is high enough (e.g., above apredetermined limit). However, when the desired rotational speed is slowenough (e.g., not above the predetermined limit) the controller isadapted to perform open-loop mode control of coil current of theelectric motor by setting the average voltage supplied to the electricmotor according to values computed from the step frequency, irrespectiveof the observed feedback current from the current feedback loop. Thepredetermined limit can be either a step frequency or a revolutionfrequency.

More specifically, the controller is adapted to perform the closed-loopmode control using feedback-based pulse width modulation (PWM) tocontrol the pattern of voltage supplied to the electric motor so as tomaintain sinusoidal current waveforms within the electric motor.

However, the controller performs the open-loop mode control to supply,from the controller to the electric motor, sinusoidal voltage waveformsmatching desired sinusoidal current waveforms corresponding to thedesired rotational speed. The current feedback loop is adapted toprovide over-current protection during the open-loop mode control.

A method embodiment receives the desired rotational speed and stepfrequency for the electric stepper motor. If the desired rotationalspeed is above a predetermined limit, the method performs closed-loopmode control of coil current of the electric stepper motor by varyingthe average voltage supplied to the electric stepper motor according toobserved feedback current from the current feedback loop connected tothe electric stepper motor. To the contrary, if the desired rotationalspeed is not above the predetermined limit, the method performsopen-loop mode control of coil current of the electric stepper motor bysetting the average voltage supplied to the electric stepper motoraccording to values computed from the step frequency, irrespective ofthe observed feedback current from the current feedback loop.

These and other features are described in, or are apparent from, thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the systems and methods are describedin detail below, with reference to the attached drawing figures, inwhich:

FIG. 1 is a graph illustrating sine and cosine current waveformssupplied to coils of an electric stepper motor;

FIG. 2 is a graph illustrating load angle within a stepper motor;

FIG. 3 is a schematic diagram of a stepper motor and controller; and

FIG. 4 is a flow diagram illustrating a methodology employed byembodiments herein.

DETAILED DESCRIPTION

While the present method and structure will be described in connectionwith embodiments thereof, it will be understood that this disclosure isnot limited to the disclosed embodiments. To the contrary, thisdisclosure is intended to cover all alternatives, modifications, andequivalents as may be included within the spirit and scope, as definedby the appended claims.

As mentioned above, the load angle shown in FIG. 2 can cause harmonicmotion. Since any imperfection causing noise at the resonant frequencycan build up this oscillation amplitude, a damping mechanism is needed.Damping occurs when velocity of the load angle causes a force that tendsto restore that velocity to zero. Damping may be accomplishedmechanically by means of tuned resonant dampers, or electromechanicallywhich is the focus of this disclosure. The feedback of load anglevelocity to acceleration force may be accomplished through a simplelocal loop, or an extravagant remote loop. This disclosure focuses on asimple local loop, since this uses less time delay and therefore has ahigher bandwidth of useful response; however, the embodiments herein areapplicable to both.

With respect to sources of vibration, if in the motor construction, theangular increment is kept small, commonly 1/64 of a cycle, and if theamplitude of the coil currents (e.g., FIG. 1) are produced accurately,then a perfect motor should run very smoothly under any constant torqueload. However, since the magnetic gaps are commonly so small andcritical, any imperfection in the stator or misalignment of the rotorwill cause deviations from the ideal waveforms, or a mismatch of thetorque per amp produced by the two coils. Even the coil winding processhas problems controlling wire resistance, and hence can cause moremismatch. It is therefore common for small motors to show coil responsesthat are mismatched by up to several percent.

The widely available Chopper Current Controlling drivers (closed-loopmode) use pulse width modulation (PWM) to reduce a high supply voltageto an average motor voltage. For a complete discussion of stepper motorcontrol circuits, see U.S. Pat. Nos. 4,587,473 and 7,154,245, thecomplete disclosures of which are incorporated herein by reference. Suchdrivers vary the supply voltage, according to current feedback, tocompensate for the back electromotive forces (EMF) that change as thevelocity varies, as required to maintain current waveforms of thecorrect amplitude. However, such conventional circuitry often encountersproblems with mismatch of current sensing resistors, voltage offsets offeedback comparators, accuracy of D/A converters, asymmetrical currentdriving, and an inherent dead band.

The “H” bridge circuit 34, shown in FIG. 3, is an output stage of adriver used for each coil of a bipolar wound stepper motor 36 and ismade up of a H pattern of transistors Q1-Q4 and diodes D1-D4. The driveruses or includes some form of central processing unit or controller 32operatively (directly or indirectly) connected to the bridge circuit 34.However, the controller 32 shown in FIG. 3 is unique because of itsability to operate the bridge circuit 34 as a closed-loop system at highspeeds and an open-loop system at low speeds.

The bridge circuit 34 is operatively connected to a power supply 30 andoutputs the appropriate voltage/current to the stepper motor 36. Acurrent sensing resistor 38 produces a feedback signal which is fed backto the controller 32 through a comparator. The feedback signalrepresents the magnitude of coil current in the direction that it isbeing driven, during the “on-time”, when either diagonal pair oftransistors (Q1; Q4 or Q2; Q3) are turned on. This is called the forwardcurrent of the motor coil, knowing that the actual direction depends onwhat is the intended direction at any given time. The actual pair ofdiagonal transistors selected determines the driving direction on thecoil.

During “off-time”, the forward coil current either does not go throughthe sense resistor 38, or flows backward through the sense resistor 38,depending on the states of the four transistors Q1 through Q4. For thisfeedback signal to be used, it is interpreted in synch with the“on-time”, and compared with a desired level, in order to determine whento switch the transistors Q1-Q4 to the “off-time” state.

When turning on a transistor in the “H” bridge driver circuit,electrical capacitance of the load and the transistor cause a currentspike to appear in the sensing resistor 38. As a result, the feedbacksignal is reversing for some time after turn-on. If turn-off istriggered by feedback levels, then the on-time is restricted to someminimum. In order to accurately control current near the zero point,using a minimum on-time, and a pre-determined off-time, current reversalduring the off-time must be prevented. This is done by using thetransistors in a combination that never applies the supply voltage inthe reverse direction during the off-time.

This causes a form of drive asymmetry where coil current variations arenormally not met by any change in the current path, but if it crosseszero when it is not expected to, the path changes. As a result, whenmotor inductance and loading effects such as back-EMF and load anglecause the current to cross zero at a different time than planned, thecircuit responds in a non-linear way, causing torque perturbations insynch with the step rate. These asymmetrical drive paths, again combinedwith the minimum on-time restriction, also explain the occurrence of“dead-band” (where the feedback loop loses control of the current at lowlevels near the zero cross). This dead band affects the motor at anyspeed, even at speeds low enough that back-EMF and inductance are notsignificant effects.

The conventional Chopper Current Controlling drives (closed-loop mode)are often unable to accurately control current at or near zero. Thisnon-linearity at the zero current crossover points cause vibration,noise, etc. One exemplary apparatus embodiment herein that providesdamping solutions to these problems comprises the dual-mode controller32 operatively connected to the electric motor 36. The controller 32 isoperatively connected, and adapted to supply different patterns ofvoltage to the electric motor 36. Further, the controller 32 has aninput/output for receiving a desired rotational speed and step frequencyof the electric motor 36. A current feedback loop is operativelyconnected to the electric motor and the controller 32.

The controller 32 is adapted to perform closed-loop mode control of coilcurrent of the electric motor 36 by varying the average voltage suppliedto the electric motor 36 according to observed feedback current from thecurrent feedback loop, if the desired rotational speed is high enough(e.g., above a predetermined limit). However, when the desiredrotational speed is slow enough (e.g., not above the predeterminedlimit) the controller 32 is adapted to perform open-loop mode control ofcoil current of the electric motor 36 by setting the average voltagesupplied to the electric motor 36 according to values computed from thestep frequency, irrespective of the observed feedback current from thecurrent feedback loop.

More specifically, the controller 32 is adapted to perform theclosed-loop mode control using feedback-based pulse width modulation tocontrol the average voltage supplied to the electric motor 36 so as tomaintain sinusoidal current waveforms within the electric motor 36.

However, the controller 32 performs the open-loop mode control tosupply, from the power supply 30 to the electric motor 36, sinusoidalvoltage waveforms matching desired sinusoidal current waveformscorresponding to the desired rotational speed. The current feedback loopis adapted to provide over-current protection during the open-loop modecontrol.

As shown in flowchart form in FIG. 4, a method embodiment receives adesired rotational speed and step frequency of an electric motor (e.g.,stepper motor) in item 400. If the desired rotational speed is above apredetermined limit, the decision box 402 directs processing to theclosed-loop mode control in item 404. In item 404, the coil current ofthe electric stepper motor 36 is controlled by varying the averagevoltage supplied to the electric stepper motor 36 according to observedfeedback current from the current feedback loop. To the contrary, asshown by the “no” result of decision box 402, if the desired rotationalspeed is not above the predetermined limit, the method performs theopen-loop mode control shown in item 406. In item 406, the rotationalcoil current of the electric stepper motor 36 is controlled by settingthe average voltage supplied to the electric stepper motor 36 accordingto values computed from the step frequency or rotational frequency inputin item 400, irrespective of the observed feedback current from thecurrent feedback loop. Item 406 is sometimes referred to as “voltagecontrol” of the motor, because the only variable monitored is thesupplied voltage pattern and the current within the motor is ignored. Initem 406, the current sensing resistor 38 only acts as an over currentprotection element.

The closed-loop mode 404 uses feedback-based pulse width modulation tocontrol the average voltage supplied to the electric stepper motor 36 soas to maintain sinusoidal current waveforms within the electric steppermotor 36. To the contrary, the open-loop mode control comprisessupplying, to the electric stepper motor 36, sinusoidal voltagewaveforms matching desired sinusoidal current waveforms corresponding tothe desired rotational speed or step frequency input in item 400. Thepredetermined limit can be a rotational speed and/or a step frequency.

Thus, the above structure and method provides a solution to commonsources of vibration by using open-loop control at slow speeds andclosed-loop control at higher speeds. At high speeds, corresponding tohigh step frequency, the inertia of the rotor presents a high impedanceto harmonic acceleration, so vibration is usually not an issue. This“fly wheel” effect allows satisfactory operation of the Chopper CurrentControlling (closed-loop) driver, and imperfect motors, but only in thehigh range of speed. However, at low speeds there is a need to overcomethe drawbacks of the Chopper Current Controlling driver, and also combatmotor imperfections with some type of dampening system. The embodimentsaddress this need by using open-loop control at low speeds. Regardingthe driver circuit in FIG. 3, the motor coil 36 could be connected tothe power supply 30 by either diagonal pair of transistors at all times.By varying the duty cycle between 0% and 100%, the average voltageapplied to the motor coil is varied from negative 100% to positive 100%of the supply voltage. A 50% duty PWM (pulse width modulation) wouldplace an average of zero volts across the coil. In this case the actualcoil current never affects the drive current path, and so the drive issymmetrical.

By utilizing open-loop control at low speeds, the embodiments hereinremove the timing restrictions that are required by having to sample thecurrent feedback signal in synch with the switching of the transistors,which may cause dead band vibration issues. No conventional feedbacksetups provide solutions for the precision feedback needed at lowspeeds. However, by controlling the current open-loop, also known as avoltage controlling drive, the dead-band problems are eliminated.

When applying a sinusoidal voltage to the coils the current is expectedto also be sinusoidal, with some time lag caused by inductance. Howeverthe current is also affected by back EMF. Because of back EMF, there isa need to increase the drive voltage amplitude in direct proportion tothe motor velocity, at a rate set by the EMF constant of the motor. Thisvoltage is added to the voltage necessary to drive the desired currentthrough the resistance of the motor coil to compensate for back EMF.

However, the back EMF varies as the load angle fluctuates. Because ofthis fact, in open-loop control, the motor velocity vibration can beelectronically dampened. In open-loop control, the generated back-EMFtends to force the coil currents to be modified in a direction thatyields a torque opposing the velocity in the load angle. This is verydifferent from the chopper current control situation (closed-loop mode)in which the current cannot be modified by back-EMF.

With the feedback loop that controls motor velocity in closed-loop mode,there are two time delays. One is the coil inductance that causes thecurrent to vary as the integral of the difference between the appliedvoltage and the back EMF. The other is the rotor inertia that causes thevelocity to vary as the integral of the current. At low motor speedsthese time delays are insignificant, and the system approximates directnegative feedback, thus stabilizing velocity. However at higher speedsthese delays become significant, to the point that feedback becomespositive, causing oscillation of velocity. For these reasons, theembodiments herein employ open-loop control at low speeds andclosed-loop control at higher speeds. Thus, the embodiments herein usevoltage control at low speeds for low vibration and high damping, anduse current control for high speeds where vibration is not an issue. Thedrive controller 32 can switch between these methods on the fly as thedesired rotational speed and step frequency change. Further, the usefulspeed ranges of these two drive methods do in fact overlap.

A standard “H” bridge circuit with a current sensing resistor (such asthat shown in FIG. 3, discussed above) may be run in either mode, simplybe altering the logic control (as shown in FIG. 4). In the case ofvoltage control, the current sense signal is not used for feedback, butcan be used as part of an over-current protection mechanism, since astalled motor will fail to produce the expected back EMF. Thus, thecurrent sensing resistor will not allow the drive voltage to causeexcessive currents and temperatures.

One feature of open-loop control (that permits very accurate currentcontrol) is that the current is very sensitive to the applied voltage.By using a symmetrical PWM driving method, very little can go wrong inthe driver that would cause current errors at low speeds. In this way atlow speeds, with open-loop control the current will be nearly ideal,except for its reaction to velocity perturbations that constitute thedesired dampening effect on vibration. With open-loop control, thetiming of the PWM signal transitions should performed accurately inorder to achieve low vibration. Fortunately, since the timing is done inthe digital realm, theoretical perfection is guaranteed.

The embodiments herein provide very good success in open-loop controlwhen the PWM frequency is set to an integer multiple (harmonic) of themicro-stepping frequency. In this way, the PWM voltage of eachmicro-step is controlled by a pulse pattern that starts and ends insynch with the micro-step time period. By locking the PWM frequency ontoa harmonic of the micro-stepping frequency in open-loop control, theembodiments herein eliminate vibrations or strange noises that appear inotherwise quiet parts of the frequency spectrum, as a result of mixingor modulation, such as beat frequencies or heterodynes.

On some examples, the range of PWM frequencies in open-loop control canrun between the top of the audio range, where coil buzzing would beobjectionable, up to the point where load capacitance and transistorswitching speed cause excessive heating and current noise. In someexamples in open-loop control, this range of PWM frequencies can bearound 18 KHz to 40 KHz, and so it spans over one octave. However, aswould be understood by those ordinarily skilled in the art, theembodiments are not limited to any ranges mentioned herein and areapplicable to all ranges, whether now known or discovered in the future.

For example, in one embodiment, the integer in open-loop control can bebased on the micro-stepping frequency and can be chosen, for any givenmotor speed, to cause the PWM frequency to fall into this predeterminedfrequency range. This could, for example allow a motor speed range ofsome ratio, such as 2:1, 4:1, etc. When the motor speed moves outsidethis frequency range, the integer can be adjusted by a factor (e.g. 2,4, etc.) such that the PWM frequency stays within its intended range atall times. The adjustment by integer powers of 2 may be performed in thehardware, by shifting the binary bits, so as to avoid the need for amultiplier unit and its associated cost. In another embodiment, therange could be set to any integer ratio (e.g. 3, 5, etc.), and the sameadjustment logic applied, but would require an integer multiplier unit.

At some point in open-loop control the motor speed becomes so high thatin the time period of one micro step, there may be very few or even justone cycle of PWM. In this situation, the controller can start skippingevery other micro step, since the micro-stepping rate may already be sohigh that the inertia of the rotor will prevent most vibration. On thecontrary, when the motor speed becomes very low, vibration may beimproved by using more micro-steps per cycle. The number of micro-stepsused in open-loop mode is limited only by the size of the sine table.

This example has provided an open-loop control with a predeterminedlimit of between 18 and 40 KHz, where the PWM duty cycle must becontrolled accurately. Again, the embodiments herein are not limited tothis example. If it is necessary to provide a resolution of 1% ofbetter, then the controller is set to count at least +/−100 increments(alternatively, 256 could be simpler) per PWM period, suggesting amaster clock of at least 8 MHz, which is commonly available. As the PWMfrequency goes down from here, to accommodate motor speed adjustments,the PWM period will occupy as much as twice the count, and theresolution will become twice as good.

As discussed in greater detail in U.S. Pat. No. 6,903,531 (incorporatedherein by reference) the duty cycle in open-loop control can be arrivedat in several ways, and can be achieved through multiplication. In thecase of a microprocessor, open-loop control should ensure thatmultiplication occurs as seldom as possible, to reduce resourceover-utilization (because in multiplication, the microprocessor needs tocalculate a new sine table every time the motor speed changes). Forexample, the processor can start with a sine template that should bescaled to reflect the motor coil resistance, such that it produces thecorrect voltages for controlling the current at zero velocity. The newtable values are the template values multiplied by one plus a speedvalue. This speed value is the motor speed, in any units, multiplied bya constant that reflects the EMF constant of the motor itself. In thisway, in open-loop control the multiplication software can be executedonly when motor speeds are changed.

Alternatively, in open-loop control the hardware can be established sothat the PWM generator free-runs with the period and duty values in itsregister, while a second counter counts the cycles. When a micro-stephas counted off, the terminal count issues a direct memory access (DMA)request that moves the next set of values from the calculated table intothe loading registers for PWM. All three of these devices repeat theiractions. The software that prepares the values would keep twodestination tables, and a speed change would be enacted simply bychanging a DMA pointer register to point to the newer of the two tables.

Using a field programmable gate array (FPGA) with embodiments hereinprovides more flexiblily by not limiting the number of motors to theresources of the microprocessor. FPGA's can also save many pins on theprocessor. Further, the motor parameters can be held in their ownregisters, making this chip completely customizable. Beyond this, FPGA'scan use the current feedback comparator as the basis for a successiveapproximation register, whereby the motor's resistance can be storedwhen first powering up the coils, and then the EMF constant can berecorded as the motor speeds up. In this way, all the user needs toselect for the application is the operating current, and the mode changespeed.

In open-loop control, when dealing with speeds that are not too low, itis possible to avoid using resources for multiplication. This is basedon the fact that at all but the lowest speeds, the coil resistance isinsignificant when compared to the back-EMF. Therefore, it is adequateto simply multiply the sine table values by the speed, and ignoreresistance. This multiplication need not consume resources, because thetable values can be directly added or subtracted from the 50% duty valuein order to establish the on-time. Since the cycle time is inverselyproportional to speed, the relative effect of the table value on theduty cycle rises with speed, and therefore is inherently multiplied.Also, since the cycle time is always twice the 50% duty value, a simpleshift left is all that is needed with this set up.

With the dual-mode embodiments described herein, the motor runs coolerin voltage mode for a given torque ability, and there is a lower averagepower requirement in voltage mode, in addition to less vibration. Theembodiments herein save the expense and bulk of mechanical dampers, andprovide a greatly extended useful speed range. These embodiments replacemechanical gear shifters, and reduce the need for speed variants inproduct inventory. They also allow direct drive, and avoid gear and/orpulley tooth vibration by eliminating their necessity.

The embodiments herein are applicable to all motors, such as steppermotors. For example, stepper motors are often used in printers or imageoutput terminals. The word “printer” or “image output terminal” as usedherein encompasses any apparatus, such as a digital copier, bookmakingmachine, facsimile machine, multi-function machine, etc. which performsa print outputting function for any purpose. The details of printers,printing engines, etc. are well-known by those ordinarily skilled in theart and are discussed in, for example, U.S. Pat. No. 6,032,004, thecomplete disclosure of which is fully incorporated herein by reference.All foregoing embodiments are specifically applicable toelectrostatographic and/or xerographic machines and/or processes and cancomprise hardware or software. For example, the method steps discussedherein can be included within a computer program product within theprocessor 32. The computer program produce could comprise acomputer-usable data carrier storing instructions that, when executed bya computer, cause the computer to perform the various method discussedherein.

It will be appreciated that the above-disclosed and other features andfunctions, or alternatives thereof, may be desirably combined into manyother different systems or applications. Various presently unforeseen orunanticipated alternatives, modifications, variations, or improvementstherein may be subsequently made by those skilled in the art which arealso intended to be encompassed by the following claims.

1. A method comprising: receiving a desired rotational speed of anelectric motor; if said desired rotational speed is above apredetermined limit, performing closed-loop mode control of coil currentof said electric motor by varying an average voltage supplied to saidelectric motor according to observed feedback current from a currentfeedback loop connected to said electric motor; and if said desiredrotational speed is not above said predetermined limit, performingopen-loop mode control of said coil current of said electric motor byvarying said average voltage supplied to said electric motor based onsaid desired rotational speed, irrespective of said observed feedbackcurrent from said current feedback loop.
 2. The method according toclaim 1, wherein said closed-loop mode control comprises usingfeedback-based pulse width modulation to control said average voltagesupplied to said electric motor so as to maintain sinusoidal currentwaveforms within said electric motor.
 3. The method according to claim1, wherein said open-loop mode control comprises supplying, to saidelectric motor, sinusoidal voltage waveforms matching desired sinusoidalcurrent waveforms corresponding to said desired rotational speed.
 4. Themethod according to claim 1, wherein said open-loop mode control furthercomprises providing over-current protection using said current feedbackloop.
 5. The method according to claim 1, wherein said predeterminedlimit comprises one of a step frequency and a revolution frequency.
 6. Amethod comprising: receiving a desired rotational speed and stepfrequency of an electric stepper motor; if said desired rotational speedis above a predetermined limit, performing closed-loop mode control ofcoil current of said electric stepper motor by varying an averagevoltage supplied to said electric stepper motor according to observedfeedback current from a current feedback loop connected to said electricstepper motor; and if said desired rotational speed is not above saidpredetermined limit, performing open-loop mode control of coil currentof said electric stepper motor by setting said average of said voltagesupplied to said electric stepper motor according to values computedfrom said step frequency, irrespective of said observed feedback currentfrom said current feedback loop.
 7. The method according to claim 6,wherein said closed-loop mode control comprises using feedback-basedpulse width modulation to control said average voltage supplied to saidelectric stepper motor so as to maintain sinusoidal current waveformswithin said electric stepper motor.
 8. The method according to claim 6,wherein said open-loop mode control comprises supplying, to saidelectric stepper motor, sinusoidal voltage waveforms matching desiredsinusoidal current waveforms corresponding to said desired rotationalspeed.
 9. The method according to claim 6, wherein said open-loop modecontrol further comprises providing over-current protection using saidcurrent feedback loop.
 10. The method according to claim 6, wherein saidpredetermined limit comprises one of a step frequency and a revolutionfrequency.
 11. An apparatus comprising: an electric motor; a controlleroperatively connected to said electric motor, said controller beingadapted to supply a pattern of voltage to said electric motor, and saidcontroller having an input/output for receiving a desired rotationalspeed of said electric motor; and a current feedback loop connected tosaid electric motor and said controller, wherein said controller isadapted to perform closed-loop mode control of said coil current of saidelectric motor by varying an average voltage supplied to said electricmotor according to observed feedback current from said current feedbackloop, if said desired rotational speed is above a predetermined limit;and wherein said controller is adapted to perform open-loop mode controlof coil current of said electric motor by setting said average of saidvoltage supplied to said electric motor based on said desired rotationalspeed, irrespective of said observed feedback current from said currentfeedback loop, if said desired rotational speed is not above saidpredetermined limit.
 12. The apparatus according to claim 11, whereinsaid controller is further adapted to perform said closed-loop modecontrol using feedback-based pulse width modulation to control saidaverage voltage supplied to said electric motor so as to maintainsinusoidal current waveforms within said electric motor.
 13. Theapparatus according to claim 11, wherein said controller is furtheradapted to perform said open-loop mode control to supply, from saidcontroller to said electric motor, sinusoidal voltage waveforms matchingdesired sinusoidal current waveforms corresponding to said desiredrotational speed.
 14. The apparatus according to claim 11, wherein saidcurrent feedback loop is adapted to provide over-current protectionduring said open-loop mode control.
 15. The apparatus according to claim11, wherein said predetermined limit comprises one of a step frequencyand a revolution frequency.
 16. An apparatus comprising: an electricmotor; a controller operatively connected to said electric motor, saidcontroller being adapted to supply a pattern of voltage to said electricmotor, and said controller having an input/output for receiving adesired rotational speed and step frequency of said electric motor; anda current feedback loop connected to said electric motor and saidcontroller, wherein said controller is adapted to perform closed-loopmode control of coil current of said electric motor by varying anaverage of said voltage supplied to said electric motor according toobserved feedback current from said current feedback loop, if saiddesired rotational speed is above a predetermined limit; and whereinsaid controller is adapted to perform open-loop mode control of saidcoil current of said electric motor by setting said average of saidvoltage supplied to said electric motor according to values computedfrom said step frequency, irrespective of said observed feedback currentfrom said current feedback loop, if said desired rotational speed is notabove said predetermined limit.
 17. The apparatus according to claim 16,wherein said controller is further adapted to perform said closed-loopmode control using feedback-based pulse width modulation to control saidaverage voltage supplied to said electric motor so as to maintainsinusoidal current waveforms within said electric motor.
 18. Theapparatus according to claim 16, wherein said controller is furtheradapted to perform said open-loop mode control to supply, from saidcontroller to said electric motor, sinusoidal voltage waveforms matchingdesired sinusoidal current waveforms corresponding to said desiredrotational speed.
 19. The apparatus according to claim 16, wherein saidcurrent feedback loop is adapted to provide over-current protectionduring said open-loop mode control.
 20. A computer program productcomprising: a computer-usable data carrier storing instructions that,when executed by a computer, cause said computer to perform a methodcomprising: receiving a desired rotational speed of an electric motor;if said desired rotational speed is above a predetermined limit,performing closed-loop mode control of coil current of said electricmotor by varying an average voltage supplied to said electric motoraccording to observed feedback current from a current feedback loopconnected to said electric motor; and if said desired rotational speedis not above said predetermined limit, performing open-loop mode controlof said coil current of said electric motor by varying said averagevoltage supplied to said electric motor, irrespective of said observedfeedback current from said current feedback loop.